What happened to the nights of the nine after the events of TES IV? Is it possible that they were disbanded by the Thalmor? or maybe the order just gradually died over time as the majority of the men started to die of old age and get too old to fight?

Any thought or answers to this?

Their mission finished, they disbanded.

wasn't part of their mandate to gather and protect the Crusaders Relics?

if their mission was "Finished", does that mean the Crusaders Relics no longer exist?

Someone from the order seemed to have traveled into Skyrim. You can find a skeleton with a flag clutching the book Knights of Nine, relatively near the Tower stone. More of a easter egg in reality, but if they made it to the Fourth Era, they were probably killed by the Thalmor after the Great War. They are the Knights of Nine, after all.

I wonder what became of the Crusaders Relics.. they are arguably some of the strongest Aedric Artifacts...

I think that's a mod I have never seen that when I play on my xbox

I remember seeing it when I played on Playstation 3.

UESP claims that the book The Knights of the Nine can be found as Dargor described. http://www.uesp.net/wiki/Skyrim:The_Knights_of_the_Nine

To the OP: I remember this question coming up not too long ago. There are a few different reasons for the Thalmor to target the Knights of the Nine, most but not all of those reasons being religious. As Dargor said, they are the Knights of the Nine, including Talos. Just as importantly, though, the purpose of the order is to protect and preserve the relics of Pelinal, the Shezarrine who was known for single-handedly slaughtering Elves wherever he found them, breaking Ayleid power in the Heartland and having a huge impact in the foundation of the Empire of Men. Regardless of the order's opinion of Talos, their foundation revolves around Pelinal, which is reason enough for the Thalmor to hate them.

And then there's the fact that the Priory of the Nine is extremely close to Cyrodiil's border with Aldmeri Dominion territory, meaning that it represented both a nearby target and a possible threat to the Dominion in general. I can't imagine that they would have just passed it by when they invaded Cyrodiil.

The Priory of the Nine is definitely the Temple of the Graal. It should have been located on Sancre Tor, though.

I like to believe the KotN came with the Divine Crusader in a Dagonite pogrom, when he was lost in the Niben and marooned on the Shivering Isles. He never found them and went terribly mad while looking...

I'm guessing the priory was targeted in the Great war and if the order even lasted after the events of TES 4: Oblivion likely the Great War or even the assassination of Ocato and ensuing chaos finished them off. Probably weren't targeted as much as the Blades which were a glorified Talos cult but they likely didn't escape the Thalmor's wrath in the Great war.